Magnetic resonance imaging (MRI) of the breast has emerged as an important new diagnostic tool for the detection of tumors. Pre-surgical MRI can help to reduce the number of biopsies that need to be taken by distinguishing benign from malignant tumors. Likewise, MRI techniques provide opportunities for staging malignant tumors that can play an important role in breast preserving surgery. More recently MRI has been employed to achieve MR-guided subcutaneous core biopsies, allowing for coordination of minimally invasive surgery. Introducing contrast agents such as gadolinium-diethylene triamine-pentaacetic acid (Gd-DTPA) during MR imaging provides sensitivity approaching 100%. An added benefit of MRI is the ability to accomplish sentinel node identification with simultaneous fat suppression. Following surgery for tumor excision, MRI is the modality of choice for the detection of residual disease. MRI is also the modality of choice for evaluating the integrity of breast implants.
The components involved in magnetic resonance imaging (MRI) include a primary magnet, computer controlled shim coils to produce field homogeneity, gradient coils to generate linear fields, radio frequencies (rf) for transmitting rf pulses and also for receiving MR imaging signals unless separate receiving coils are integrated and advanced specialized software for data acquisition, analysis and pulse sequence. MR imaging involves exposing nuclei to a strong magnetic field and then excitation by rf resonant energy.
Despite the sensitivity MR imaging provides over more traditional imaging modalities such as mammography or ultrasound, there are several technologic hurdles. While providing exceptional sensitivity, it displays variable specificity that represents a major limitation. An area of concern given the varying specificity is the patient management decisions when tumors are detected by MR imaging that can not be identified with more traditional imaging modalities.
To aid enhanced MR imaging of the breast, MRI equipment manufacturers have developed and marketed breast coils. Breast coils are usually whole-volume solenoids used for transmission and receiving. Resonators are applied over the breast, usually in pairs to allow for simultaneous imaging of both breasts. The quality of the MR image produced can be enhanced by the optimal use of an independent coil placed close to the area being imaged in order to improve the strength of any received signal and are called surface coils. Many of the breast coils are system/manufacturer specific. While the aperture configuration and dimensions of the various breast coils may vary slightly, all breast coils presently in commercial use, receive (position) the breasts of women while lying face down and can image both unilaterally and bilaterally. RF coils designed specifically for imaging breast tissue are described in more detail, for example, in U.S. Pat. Nos. 6,850,065, 6,198,962, 6,163,717, and 5,706,812.
For example, a breast coil designed by MRI Devices Corporation and marketed by General Electric uses an open design with a sternum pad that provides lateral access to either breast for patient positioning. The General Electric employs dual apertures for imaging breasts with a rectangular configuration of approximately 4.75 inches by 7.25 inches to position the breasts of women. Not being limited by the view provided by a lateral window, the hanging breasts can be viewed from all angles.
Similarly, Toshiba Medical Systems markets a breast array coil that can be used to image the female breast, chest wall and axillae. Toshiba's breast coil uses a four-channel phased array in the receive mode exclusively. A side window in the breast coil allows visualization of the exposed breasts by the MRI technician to ensure proper placement of the breasts within the breast coil.
The Siemens Medical Systems breast array coil also lies directly on the table within the magnetic field. Similarly, the design also allows for visual control of the position of the breasts within the breast coil through a transparent window on the side of the coil that is performed by the MRI technician. The four-coil design with four independent preamplifiers employ stability pads that are located on the sides of the coil apertures that are used to compress the breast tissue and to adjust for wide variation in anatomical dimensions.
The breast coil of Hitachi Medical Systems is similar in design to both those of Toshiba and Siemens. It is a quatrature coil for bilateral breast, chest wall and auxiliary node imaging. Similar to the other commercially available breast coils, the woman lies face down with pendulantly suspended breasts positioned in the two coil apertures and compressed to accommodate for varying breast size by the technician using visual access.
Philips uses a 4-channel phased array breast coil for both unilateral and bilateral imaging of the breast, chest wall and axillary tissue. The 4-channel array design uses open architecture that allows for interventional access with easy biopsy and needle placement. The Philips coil uses oval apertures of 16 cm diameter and 12 cm depth to accommodate the variations in breast size.
In using all of the above mentioned breast coils, the woman lies prone face down and head first. The breasts must be positioned in the center of the coils. Since there is a tendency for patients to slide too far superiorly in the coil, they are usually asked to slide towards their feet after lying prone on the coil support. The various breast coils used clinically are all made of hard molded plastics. Since the breasts are suspended pendulantly, below the surface of the breast coil, the weight of the patient is placed on the chest bones. To make the procedure more comfortable a high resilience, high density foam pad with the same diameter and aperture configuration of the coil is placed on the top surface of the coil. The patient lies on top of the coil pad. However, the coil pads are 2-dimensional and do not provide image enhancement by either fat saturation or by reducing the skin to air interface.
The salient feature common to all of these breast coils is the need to adequately position women's breasts that are hanging pendulantly. Another common feature of the breast coils in proper positioning breasts within the coil is the aspect of using some form of compression to immobilize the breasts and adapt the apertures for variation in individual breast size. These “one size fits all” breast coils rely on filling the residual volume between the breast tissue aperture space manually by the MR imaging technician that is visibly guided and manual (FIG. 1). This residual space is filled using materials placed within the coil aperture such as dielectric pads or by compression with plastic spacers.
The non-uniformities in the magnetic resonance system's main magnetic field that is generated by the variations in breast geometry and magnetic susceptibility in localized areas of the breast can be problematic in obtaining good quality images. Fat suppression is an imaging technique that relies on changing the relative brightness of fat in comparison to water in order to obtain a better quality image. However, the fat saturation techniques used in magnetic resonance imaging are highly sensitive to homogeneity in the magnetic field and breast anatomy is not homogenous in areas. Specific areas of concern are the areas where the breast and chest wall interface. The techniques attempt to selectively excite fat with a narrow band RF pulse that has the characteristic resonance frequency of fat, while leaving water unaffected. Fat saturation techniques while beneficial, may not uniformly suppress the fat within the breast tissue being imaged and can result in cross-suppression where water molecules may be likewise affected. The result is unreliable fat saturation with a less than optimal image.
Another method for controlling the homogeneity of the magnetic field includes both passive and active shimming techniques. The passive techniques is typified by arranging steel shims to minimize static magnetic field inhomogeneities at diameters comparable to the gradient coils. A major shortcoming of passive shimming is not adjustable from scan to scan.
Active shimming uses shim coils that can be adjusted to establish uniform magnetic fields by integrating the localized shim coil into a RF coil. The localized shim coil can then be engaged during a fat saturation pulse sequence that is being transmitted by the magnetic resonance system. While contributing to image enhancement, shimming may not uniformally suppress fat and can suppress regions of water in the breast.
Another method to enhance fat saturation during magnetic resonance imaging of the breast is the use of a flexible bag containing a variable amount of a fat saturation enhancing material that surrounds a flexible coil. U.S. Pat. No. 5,414,358 describes a bag made preferably of polyurethane and filled with a perfluorochemical liquid. The breast bag is connected to an inlet pipe which has a pump and an outlet pipe having a shut-off valve. A reservoir is connected to the opposite ends of both the inlet and the outlet pipes. When the bag is emptied of the perfluorochemical liquid a hollow center forms into which breast can be inserted. The bag is then refilled by opening the inlet valve, then turning on the pump to feed the perfluorochemical liquid into the inlet pipe from the reservoir and at the same time closing the valve in the outlet pipe. The flexible bag is secured to the coil and the patient by means of straps. There are several significant drawbacks to this device. First, it is not compatible for use with any of the breast coils being used clinically. The breasts placed in the breast bag cannot be further placed into the breast coil apertures since the base of the breast bag is contiguous and flat. Secondly, the volume of fat saturation enhancing material that is re-introduced is variable without intrinsic controls to ensure there is enough material present to provide adequate image enhancement. Thirdly, the breasts are compressed by the weight of the patient laying prone on the breast bag and subsequently further compressed when the bag is refilled with the perfluorochemical liquid after being inflated. Hence, the breasts are compressed from two sides by the weight of the woman and by the simultaneous pressure of the perfluorochemical in the inflated breast bag. The resulting compression of the breast tissue negatively impacts the ability to image the vasculature and lesion angiogenesis. Fourthly, the selectively fillable breast bags covers the breasts on all sides making biopsy impossible. The opportunity to perform the biopsies that are an integrated component of the breast coils is eliminated.
U.S. Pat. Nos. 5,414,358 and 5,339,033 issued to Eilenberg et al. describe a method for improving fat saturation during MRI. The method uses a pad containing a fat saturation material, where the pad is applied to the anatomy to be imaged.
Interventional MRI procedures typically require that an MR signal detection coil have large openings so that a surgeon can have access to the surgical site through the coil with a biopsy needle or other surgical devices. However, in the absence of a fat saturation device, artifacts are common. The difficulty in providing surgical access is especially difficult for breasts, due to the varying sizes and shapes and desirability of minimizing manipulation which is stressful for the patient. What is needed is a fat saturation device specifically designed for use in conjunction with breast coils.